A switching power supply of a kind that has previously been proposed is shown in FIG. 1 and comprises a transformer 2 having its primary winding 4 connected in series with the collector-emitter path of a transistor 6 between the two terminals of a DC voltage source, and a secondary winding 8 connected through a rectifier 10 and a low pass filter 12 to a load 14. The load might be, for example, the power supply bus of a laboratory instrument. It will be understood that by those skilled in the art that when the transistor 6 is switched on, current builds up in the primary winding 4. When the transistor 6 is turned off, the current switches to the secondary winding 8, where it decays. The current flowing in the secondary winding 8 of the transformer 2 is rectified and filtered and is applied to the load 14.
The transistor 6 is turned on and off by a base drive circuit comprising a transformer 16, a transistor 18, a zener diode 20 and a pulse generator 22. Typically, the primary winding 24 of the transformer 16 is connected to a +12 volts DC source. The pulse generator 22 generates pulses in a pulse train having a repetition frequency that would typically be about 30 kHz. The duty cycle of the pulse train varies in dependence upon the power demands of the load connected to the transformer 2. Typically, the duty cycle might vary between 10% and 50%. When the transistor 18 is switched on by a pulse from the generator 22, current flows in the primary winding 24 of the transformer 16 and a corresponding current is induced in the secondary winding 26, providing forward base current to the transistor 6 and thus turning the transistor 6 on, and resulting in build-up of charge in the base-emitter region of the transistor 6. The forward base current is limited by the resistor 28. During the time that the transistor 18 is on, magnetizing current is building in the primary winding 24 of the transformer 16.
When forward base current flows in the secondary winding 26, the capacitor 30 is charged in the manner indicated, so that when current ceases to flow in the primary winding 24 the base of the transistor 6 is held negative, thus keeping the transistor 6 off. The diode 32 limits the potential drop across the capacitor 30 to one diode drop (about 0.6 volts).
When the transistor 18 is turned off, the voltage at the collector of the transistor 18 rises somewhat, e.g. to 13 volts, and the magnetizing current which was flowing in the primary winding 24 of the transformer 16 is transferred to the secondary winding 26 as reverse base current, drawing charge carriers from the base-emitter region of the transistor 6. The reverse base current is proportional to the primary winding's magnetizing current. When the transistor 6 can no longer sustain reverse base current (its base-emitter region is depleted of charge carriers and it therefore turns off), current ceases to flow in the secondary winding 26. At that point, the magnetizing current is transferred back to the primary winding 24 and the voltage at the collector of the transistor 18 rises to the threshold value of the zener diode 20, e.g. 30 volts. The large potential difference across the primary winding 24 allows rapid demagnetization of the transformer core.
The sequence of events happens once per cycle at the operating frequency of the generator 22.
As noted above, the reverse base current is proportional to the magnetizing current that flows in the primary winding 24. The magnetizing current is proportional to the length of time that the transistor 18 is on (duty cycle).
The reverse base current is dependent on duty cycle, and since duty cycle changes with input voltage and output load conditions, the reverse base current can change substantially. This implies that the transistor 6 may dissipate more power than necessary under certain line and load conditions.